Lithium metal has a variety of industrial uses, including applications for aluminum-lithium alloys. Additional uses of this metal include lightweight, compact primary and secondary lithium batteries, compact lithium/sulfur batteries for electric cars and power plant load leveling products Lithium metal is also employed as a degasifier in the production of high-conductivity copper and bronze and in the synthesis of organometallic compounds for applications in the fields of rubber, plastics and medicines.
Lithium metal is generally produced by electrolysis of an eutectic mixture of highly pure molten lithium chloride and potassium chloride. The lithium chloride for metal applications has been conventionally produced according to several methods. In one method lithium chloride is produced by directly reacting hydrochloric acid with lithium carbonate produced from spodumene as its genesis. However, lithium chloride produced in this manner contains substantial contaminants which are not suitable for many of the present applications of lithium metal.
To obtain highly purified lithium chloride suitable for all lithium metal applications, another method involves converting lithium carbonate to lithium hydroxide. This process requires considerable effort to generate a highly pure form of lithium hydroxide. Lithium hydroxide produced in this way is subsequently reacted with purified hydrochloric acid and evaporated to dryness to produce highly pure lithium chloride. While this method is technically viable, it is far more costly than yet another method involving the production of lithium chloride directly from naturally occurring lithium chloride brines.
Direct production of high purity lithium chloride from brine, however, is not a simple process. Naturally occurring brines found, e.g., in the United States and Chile, contain reasonable concentrations of lithium, in the form of lithium chloride Some of these brines have high concentrations of lithium and a low magnesium to lithium ratio, generally about 1:1 to 6:1, which allow for concentrating, purifying and recovering lithium chloride brine While these brines are viable reserves for lithium recovery, they also contain varying amounts of alkali and alkaline earth metal impurities, such as, magnesium, calcium, sodium, sulfate, boron, and other components. Some typical components of naturally occurring brines are identified in the Table below entitled "Saline Brine Analyses".
TABLE __________________________________________________________________________ SALINE BRINE ANALYSES Weight Percent Great Geothermal Silver Atacama Dead Sea Salt Lake Bonneville Salton Sea Peak Brine Ocean Israel Utah Utah California Nevada Chile __________________________________________________________________________ Na 1.05 3.0 7.0 9.4 5.71 6.2 7.17 K 0.038 0.6 0.4 0.6 1.42 0.8 1.85 Mg 0.123 4.0 0.8 0.4 0.028 0.02 0.96 Li 0.0001 0.002 0.006 0.007 0.022 0.02 0.15 Ca 0.040 0.3 0.03 0.12 2.62 0.02 0.031 SO.sub.4 0.25 0.05 1.5 0.5 0.00 0.71 1.46 Cl 1.900 16.0 14.0 16.0 15.06 10.06 16.04 Br 0.0065 0.4 0.0 0.0 0.0 0.002 0.005 B 0.0004 0.003 0.007 0.007 0.039 0.005 0.04 Li/Mg 1/12720 1/2000 1/135 1/60 1/1.3 1/1 1/6 Li/K 1/3800 1/300 1/70 1/90 1/71 1/20 1/12 Li/Ca 1/400 1/150 1/5 1/17 1/119 1/1 1/0.2 Li/B 1/4 1/1.5 1/1.2 1/1 1/1.8 1/0.25 1/0.27 __________________________________________________________________________
These metal ion contaminants in lithium containing natural brines, should be substantially eliminated or minimized to produce a lithium chloride product suitable for production by electrolysis of an uncontaminated lithium metal. During electrolysis of lithium chloride to produce lithium metal, contaminating metal ions report to the lithium metal due to the high electrode potential required for reduction of lithium. Also any contaminating anion in the lithium chloride which is not oxidized at the anode to form a volatile species will build up in the electrolyte and eventually cause substantial losses in current efficiency. For example, estimates have been made that concentrations in excess of about 100 ppm of borate ion or about 25 ppm of boron present in the lithium chloride produced via brine production technology are not satisfactory for long term electrolytic cell operation.
Simple technical means for the removal of all of these contaminants from the ultimately produced lithium metal or from the lithium chloride brines are currently not cost effective. As presently practiced in the industry, boron is removed from, or substantially reduced in, lithium chloride brine on a commercial basis by first converting the lithium chloride brine containing substantial impurities into lithium carbonate via a process of precipitation of lithium carbonate with soda ash. The lithium carbonate is subsequently converted to lithium hydroxide by lime treatment of lithium carbonate to produce lithium hydroxide and waste calcium carbonate. Crystallization of the lithium hydroxide substantially removes the boron and alkali metal impurities by way of a bleed stream. The lithium hydroxide is then treated with hydrochloric acid to produce lithium chloride or treated with CO.sub.2 to produce high purity lithium carbonate. These conversions are accomplished according to the following series of reactions: EQU 2LiCl+Na.sub.2 Co.sub.3 .fwdarw.2NaCl+Li.sub.2 CO.sub.3 ( 1) EQU Li.sub.2 CO.sub.3 +Ca(OH).sub.2 +2H.sub.2 O.fwdarw.2LiOH.H.sub.2 O+CaCO.sub.3 ( 2) EQU LiOH.H.sub.2 O+HCL.fwdarw.LiCl+2H.sub.2 O (3) EQU 2LiOH.H.sub.2 O+CO.sub.2 .fwdarw.Li.sub.2 CO.sub.3 +3H.sub.2 O (4)
This process, while both complex and costly, is conventionally utilized to obtain lithium chloride of sufficient purity for use in the electrolytic production of lithium metal from lithium chloride.
Lithium carbonate crystals precipitated from lithium chloride brines containing boron typically retain a contaminating quantity of lithium borate. One of the problems associated with borate ion in lithium brines is its solubility up to relatively high concentrations of 5,000 to 10,000 ppm as boron and four to five times that weight as borate, depending upon the particular borate ion species present (species vary with pH). For example, typical commercial lithium carbonate produced from brines at Silver Peak, Nevada and Chile contain approximately 400 ppm of boron, where the original lithium concentration was approximately 7,000 ppm and the boron concentration approximately 2,000 ppm. Lithium carbonate produced from such brines normally retains the borate contaminant. Therefore, precipitation of lithium carbonate is not an adequate means by which boron can be excluded from the resultant lithium salt.
A number of additional methods for boron removal have been used in the field of lithium metal manufacture. Among such methods include treatment of a brine with slaked lime to precipitate calcium borate and/or, where brines contain substantial magnesium impurities, magnesium borate. Attempts to absorb borates on clays, on HCO.sub.3 -- and Cl type resins, or on activated alumina in the presence of magnesium have also been employed to reduce the boron content of brines. Another unsatisfactory method is precipitating borate as a borophosphate concentrate by treating the brine with lime in combination with phosphoric acid. Brines have also been acidified to precipitate boric acid, and treated by solvent-solvent extraction, i.e. with n-butanol. None of these methods have proven to be cost effective for widespread commercial application.
As an example of efforts extant in the art to remove boron from lithium-containing brines, U.S. Pat. No. 4,261,960 discloses the removal of boron, as well as magnesium and sulfate, by treatment of the brine with an aqueous slurry of slaked lime and an aqueous solution of calcium chloride, followed by concentrating and eventual calcination.
In addition, other methods for removal of borates and boric acids from brines of Searles Lake are described in D. S. Arnold, "Process Control in Boric Acid Extraction", Metallurgical Society Conferences, Vol 49, 125-140, Gordon and Breach, Science Publishers, New York, (1968) [Hydrometallurgy Session, Second Annual Operating Conference, The Metallurgical Society of AIME, Philadelphia, Pa., Dec. 5-8, (1966)]. See, also, D. S. Arnold, "A New Process for the Production of Boric Acid", 19th Annual Technical Meeting, South Texas Section of AIChE, Galveston, Texas (October 23, 1964); C. R. Havighorst, "Kirkpatrick Award Winner/AP&CC's New Process Separates Borates from Ore by Extraction", Chemical Engineering, 70, 228-232 (Nov. 11, 1963. In addition other publications and U.S. patents on boric acid extraction date from 1960 on, including U.S. Pat. Nos. 2,969,275; 3,111,383; 3,297,737; 3,336,115; 3,370,093; 3,436,344; 3,479,294 and 4,261,961.
Other processes have been developed to remove boron from lithium chloride generated from brine sources. For example, U.S. Pat. No. 3,855,382 discloses the extraction of boron from magnesium chloride solutions using a solution of iso-octanol in petroleum ether. These processes are nevertheless costly for production or purification of lithium chloride with boron concentrations less than 25 ppm B. U.S. Pat. No. 4,271,131 and related U.S. Patent Nos. 4,243,392; 4,274,834; 4,261,960 teach the concentration of lithium chloride to about 40% by weight, followed by heating at high temperatures in excess of 200.degree. C. to insolubilize the boron in an isopropanol solution which extracts purer lithium chloride, followed by evaporation of the isopropanol and crystallization of the lithium chloride. These processes involve a calcination step which is costly in terms of the operating cost and capital investment, due to the required materials of construction. In addition, yield losses are observed which increase the overall cost of the product.
There remains, therefore, a need in the art for a satisfactory, cost-effective method for production of a boron-free, highly pure lithium chloride from lithium-containing brines, which is satisfactory for electrolysis directly to lithium metal.